[2] Usually the performance of a diagnostic test is measured in terms of its sensitivity and specificity and it is changes in these that are considered when referring to spectrum bias.
[2] The first is due to a change in the case-mix of those patients with the target disorder (disease) and this affects the sensitivity.
The second is due to a change in the case-mix of those without the target disorder (disease-free) and this affects the specificity.
[4] This final cause is not widely appreciated, but there is mounting empirical evidence[4][5] as well as theoretical arguments[6] which suggest that it does indeed affect a test's performance.
[9] If properly analyzed, recognition of heterogeneity of subgroups can lead to insights about the test's performance in varying populations.